The present invention relates to a direct-to-plate method for making a heat-sensitive high quality lithographic printing plate by means of an ink-jet printhead.
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas, which accept ink, form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made image-wise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive.
Heat-sensitive imaging elements having an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a water or alkali soluble or swellable resin and a compound capable of converting light into heat, are disclosed in e.g. EP-A-770 494, EP-A-770 495, EP-A-770 496, EP-A-770 497, EP-A-773 112, EP-A-773 113, EP-A-774 364, EP-A-800 928, EP-A-96 202 685, EP-A-96 203 003, EP-A-96 203 004 and EP-A-96 203 633. In most cases carbon black or an IR-dye are mentioned as the compound capable of converting light into heat.
In order to prepare an imaging element as described above, that is processable on the press, preferably IR-absorbing compounds are used. Both carbon black and IR-absorbing dyes or pigments can be used.
The coatings which are used for the preparation of lithographic printing plates are mostly applied with coating techniques such as dipcoating, cascade coating and curtain coating. When the coating proceeds on the press, spray techniques are very often used. The term xe2x80x9csprayxe2x80x9d means a method comprising spraying an aqueous solution or dispersion of a resin (coating solution) by a spraying device to atomize the coating solution and adsorbing and coating the coating solution sprayed onto a substrate. Spray techniques for applying layers on a substrate that is placed on a printing press are disclosed in e.g., U.S. Pat. No. 4,626,484, EP-A-818 711, U.S. Pat. No. 5,713,287, etc.
Several spraying techniques are known in the art: the air-spraying method comprises using a compressed air as spraying manner. The airless spraying method (liquid pressure spraying method) comprises spraying the coating solution by applying high pressure to the coating solution. The electrostatic spraying method comprises spraying the coating solution by electrostatic force. The electrostatic coating method is advantageous to increase adhesion efficiency of the resin in the coating solution and comprises employing an earthed article to be coated as an anode and a coating solution spraying device as a cathode, forming an electrostatic field between the anode and the cathode by applying high negative voltage, negatively charging the resin particles and adsorbing the resin particles on the article to be coated with good efficiency.
The use of airless spray techniques, however, for applying lithographic coating solutions makes it quite difficult to achieve high cosmetic quality (mainly evenness of the coating without stripes or bands) of the end product. The conditions for lithographic materials of high quality (thermal printing plates processable as well as not processable on-press) where high resolution, sensitivity and reproducing characteristics are required, are very high with relation to the cosmetic quality of said printing plate. This cosmetic quality can be translated as the presence of lines, the general evenness and the presence of a mottle pattern.
Air assisted spraying of lithographic coating solutions improves the cosmetic quality of the end product. Lithographic printing plates prepared by using this technique are described in EP-A 1084380 and EP-A 1084862 both filed on 15.09.1999. However, with this technique the coating solution is highly atomized resulting in a low yield of coating solution on the lithographic substrate since up to 50% of the atomized coating solution is wasted into the environment. Furthermore there is a high risk of contaminating printing press elements such as ink rollers, imaging units, cleaning units.
Thus a technique for applying layers on a substrate that is placed on a printing press that makes it possible to have a layer without mottle, banding and that does not soil the printing press is still highly desirable.
It is an object of the present invention to provide a direct-to-plate method for obtaining a high quality lithographic printing plate avoiding waste of coating solution and contamination of printing press elements. It is also an object of the present invention to provide a direct-to-plate method wherein the lithographic substrate can be reused.
The above objects are obtained by the method defined in claim 1.
The high quality lithographic printing plate of the present invention can be obtained by applying a continuous layer of a radiation sensitive solution onto a lithographic substrate by means of an ink-jet printhead whereby small drops of coating solution are jetted directly onto the lithographic substrate. Hereby no coating solution is wasted into the environment. Furthermore there is no risk of contaminating other elements of the printing press such as the ink rollers or the imaging unit.
Further advantages and preferred embodiments of the present invention will become apparent from the description hereinafter.